Current design standards for United States hydrologic infrastructure may be inadequate to deal with the increasing frequency and severity of extreme rainstorms. Structures like retention ponds and dams could face more frequent and severe flooding, according to a new study published in the AGU journal Geophysical Research Letters.

The research analyzed data from multiple regions of the U.S. and found that the rising number of extreme storms combined with outdated building criteria could overwhelm hydrologic structures like stormwater systems.

Sunny day tidal flooding in downtown Miami. Credit: Wikimedia Commons"The take-home message is that infrastructure in most parts of the country is no longer performing at the level that it's supposed to, because of the big changes that we've seen in extreme rainfall," said Daniel Wright, a hydrologist at the University of Wisconsin-Madison and lead author of the new study.

Engineers often use statistical estimates called IDF curves to describe the intensity, duration and frequency of rainfall in each area. The curves, published by the National Oceanic and Atmospheric Administration (NOAA), are created using statistical methods that assume weather patterns remain static over time.

100-year storms

But extreme rainfall events are occurring more often in many regions of the world, something IDF curves do not take into account. One measure of extreme rainfall is the 100-year storm, a storm that has a 1% chance of happening in a given year, or a statistical likelihood of happening once in 100 years on average.

Wright and his colleagues wanted to know how existing IDF curves compare with recent changes in extreme rainfall. They analyzed records from more than 900 weather stations across the U.S. from 1950 to 2017 and recorded the number of times extreme storms - like 100-year storms - exceeded design standards.

They found that in the eastern United States, extreme rainstorm events were happening 85% more often in 2017 than in 1950. In the western U.S., extreme storms are appearing 51% more often over the same time period.

The scientists found that in most of the country, the growing number of extreme rainstorms can be linked to warming temperatures from climate change, although natural events, such as El Niño, also occasionally affect the Southeast's climate.

By comparing the number of storms that actually happened against the number predicted by IDF curves, the researchers found that in some regions infrastructure designed to withstand extreme rainstorms could face these storms every 40 years instead of every 100 years.

"Infrastructure that has been designed to these commonly-used standards is likely to be overwhelmed more often than it is supposed to be," Wright said.

The researchers said they hope the findings will encourage climate scientists, hydrologists and engineers to collaborate and improve U.S. hydrologic infrastructure guidelines.